Two-photon lithography is a very powerful, yet simple technique to produce complex, three-dimensional structure from a liquid, photosensitive material. Two-photon polymerization (TPP) is based on the simultaneous absorption of two photons, which induce chemical reactions between starter molecules and monomers within a transparent matrix. The absorption of 2 photons requires extremely high peak intensities, thus an ultra-short pulse laser is needed to provide the high intensity. Previously, the most common application of two-photon absorption (TPA) has been two-photon confocal microscope where the fluorescence of a dye molecule is observed after being excited by the means of TPA. Single-photon absorption used in standard photo- and stereolithograpic techniques is inherently two-dimensional, since ultra-violet light is absorbed by the resin within the first few micrometers. Since the photosensitive resins are transparent in the near-infrared (NIR) region, NIR laser pulses can be focused into the volume of the resin. As the laser focus is moved three-dimensionally through the volume of the resin, the polymerization process is initiated along the path allowing the fabrication of any 3D microstructure.
The rate of TPA is non-linear or quadratic dependent on incident intensity, therefore making it possible to achieve lateral resolutions better than 100 nm in the polymerized structures. For many applications that requires 3D structures, such as tissue engineering scaffolds, biomedical implants, micro-lens, micro optics and other micro devices (MEMS) requires 3D resolutions in a few microns, the TPP process offer a fast and simple way to achieve the desired resolutions.
Nanoimprint Technology
The principle of nanoimprint is straightforward. A schematic of the process developed in the original NIL process is shown in FIG. 15. A hard mold that contains micron-nanoscale surface relief features is pressed into a polymeric material cast on a substrate at a controlled temperature and pressure, thereby creating a thickness contrast in the polymeric material. A thin residual layer of polymeric material is left underneath the mold protrusions and acts as a soft cushioning layer that prevents direct impact of the hard mold on the substrate and effectively protects the delicate nanoscale features on the mold surface. For most applications, this residual layer needs to be removed by an anisotropic O2 plasma-etching process to complete the pattern definition.
A variation of nanoimprint has also been developed known as Step and Flash Imprint Lithography (SFIL) or UV nanoimprint lithography. In this technique, a transparent mold and UV-curable precursor liquid to define the pattern is used, allowing the process to be carried out in room temperature, as illustrated in FIG. 16.